Mister Transistor's Historic Semiconductors

Perhaps the most famous name in English electronic components is that of Mullard Limited. This company was founded by Captain S.R. Mullard in 1920 to manufacturer high-power transmitting valves for the British Admiralty. (For American readers, I had better explain that in Britain an electronic 'valve' is what you call a 'vacuum tube'). His business prospered and in 1924 he raised capital by selling half the company to Philips of the Netherlands, who bought the rest of the company in 1927. Wholly owned by Philips, Mullard Ltd. went on to become a major manufacturer of valves and associated components.

Many valve enthusiasts have created Web sites that cover Mullard's valve development but information about their semiconductor development is scarce. Mullard's original manufacturing facilities were in London, and their head office remained there, but I believe that their semiconductor production was located in Southampton. I am unsure whether their research was based there, or at Philips laboratories in Eindhoven. If you know about this, or have other information about Mullard's early semiconductors, please

When the transistor was announced by Bell Labs in 1948, two UK companies were not far behind in research terms, STC and GEC. Mullard had no semiconductor research laboratory as far as I know, and I suspect that Philips also had done little work in the field. However, a former Mullard employee told me that Philips made the decision in about 1950 to start manufacturing transistors, with the goal of capturing 95% of the European market. I do not have statistics, but the Mullard brand certainly became dominant very quickly, and remained so for nearly twenty years.

Mullard's main semiconductor product lines were the OC series of germanium transistors and the OA series of germanium diodes,. although they also used many other prefixes such as OAZ, AA, AC, AD, AF, ASY, ASZ, AUY, and BC. The OC series was very widely used in Europe for a period of almost twenty years and many millions were made. In the USA, they were sold by Amperex. In Germany, Valvo was another subsidiary of Philips that made the same devices, and Intermetall and Siemens made their own OC types.

True Mullard devices usually bear the name MULLARD in capitals, the type number e.g. OC71, and an indication of the place of manufacture, usually either "BRITISH MADE" or "GREAT BRITAIN". These two phrases seem to have been used equally, if you know any difference in their significance, I would be interested to hear. Occasionally examples can be found that are marked "MADE IN HOLLAND", and very rarely "MADE IN FRANCE". Examples can also be found that bear no manufacturer's name or place of origin, I believe these to be made by Philips in the Netherlands. They usually have a code printed on them, composed of a small number of mixed-case letters and numbers. I do not know how to decode this, or even whether it is a date code or something else. If you can help, please contact me.

I do not possess all the transistors below, in fact I am seeking many of them. It is noted in the text where I am seeking examples of any particular type: if you have some for sale or exchange, please Conversely, I am happy to help anyone looking for information on these devices. I have an extensive collection of original data sheets and books, and can provide characteristic data on almost all OC series types.

The first transistors made by Mullard were the OC50 and OC51 point-contact types from 1952. As this image of the OC51 shows, they looked just like a typical Bell Labs "outline #1" case, except that the metal is shinier, and the plastic plug in which the leads are fixed is pale brown. I have two of them and neither has any markings on it, rather disappointingly. This transistor was probably not commercially available: it was shown to a few research institutes, then Mullard rapidly moved on to junction types.
I am seeking examples of the OC50, and indeed of the OC51 in identifiable form, for example in an original box. I also wish to buy original data sheets for these devices.

The first (grown?) junction types made by Mullard were the OC10, OC11, and OC12 (shown in the image) from 1953. These were all low-power audio-frequency amplifiers, the first two differing in noise level, the OC12 having a higher gain. Unfortunately, it was soon found that the plastic encapsulation was not hermetic, and moisture crept along the leads and ruined the transistors.
I am seeking examples of the OC10, OC11 and OC12. I also wish to buy original data sheets for these devices.

Mullard then developed a unique glass encapsulation, and in 1954 issued three new alloy-junction audio-frequency transistors, the OC70, OC71 and OC72. The image shows the OC71 glass case, painted black, with below it the OC72, a higher power type, achieved by slipping an aluminium can over the glass tube. The OC70 was a low-noise version of the OC71.

This black glass encapsulation, and the version with the aluminium sleeve, both referred to as SO-2, were used by most of the transistor types in the OC series, although a few types used the TO-7 metal can, and power types used TO-3. A correspondent told me that in the early days of the industry, an apprentice at his company was sacked because he had amused himself by crunching the glass capsules under his boot. At the time each transistor cost a week's wages!

Mullard produced the first true high-power transistor in Europe, the OC16, in about 1956. The prototype version on the left in the image handles 24 Watts. The commercial version to its right used a different stud packaging in which two leads emerged through a hole in the centre of the fixing screw. This package was only used for the OC16, apart from two exceptions that I know: I have a single OC24 in this package (normally that device uses the TO-3 package), and I possess a strange Mullard type CXT6b, shown below.

Mullard soon abandoned the proprietary stud encapsulation for high-power transistors, and changed to using the standard TO-3 'diamond' package, as shown in this OC19, another audio-frequency type from about 1962. At this time, Mullard were still supplying their transistors in the same kind of cartons that they used for valves.

Also in 1956, Mullard announced the OC44 and OC45 transistors, the latter seems to be described all over the Web as "the first RF transistor", although I think this only applies to Europe. In reality these two devices would be more accurately termed IF transistors. There is an ongoing market for the OC45 on eBay because it was used in some early guitar effects units ("fuzz boxes") and purists insist that only the original will do. There are even people who talk about the "germanium sound", and do not mean poor signal-to-noise ratio!

Like other manufacturers, Mullard believed that hearing aids provided an early application for the transistor, because of its small size, low voltage, and lack of heater circuit compared to valves/tubes. This was ultimately only a minor market, but Mullard created four sub-miniature types, OC57 to OC60, only a few millimetres long, and two miniature types OC65 and OC66, slightly larger. All were audio-frequency amplifiers for use in the output stages of hearing aids.

The first RF transistors used the alloy-junction fabrication process, which limited their upper-frequency limit. Mullard soon followed the American lead in improving upon this with a technique called alloy-drift, where controlled doping gradients were used. Their first examples were the OC169 (shown) to OC171 in 1959, but later their AF114 to AF118 types from 1961 onwards, using the same TO-7 metal can with shield lead, became standard in the RF stages of British transistor radios.

Mullard made a small number of devices that were intended as driver transistors for audio output stages. These bore the suffix 'D', and examples include OC78D, OC81D, and OC82D. For reasons unknown to me, these types often used the SO-2 glass capsule but with a coloured plastic sleeve, rather then being painted black. This is occasionally seen on other types too. If anyone can explain this, please contact me.

Of course, Mullard also produced switching transistors for use in computers and logic circuits. In 1959 they announced a number of these, including the OC41 and OC42 in the SO-2 metal can, and the rather unusual OC139 to OC141 which are germanium NPN types in the SO-2 glass package, usually marked "MADE IN HOLLAND". All of these devices found widespread use in the UK electronics industry, including computer manufacture.

Mullard's first silicon transistors were the OC201 to OC206, PNP alloy types using the standard SO-2 metal-over-glass construction. These can occasionally be found on eBay, usually advertised as germanium devices! Mullard soon switched to using the BC prefix for silicon, and AC for germanium, eliminating the confusion. Only a few of the BC types are found in the SO-2 metal packaging, most used newer plastic encapsulation.

Like other manufacturers, Mullard produced a phototransistor, the OCP71 shown on the left. It was widely believed that this was in fact just an ordinary OC71 without the black paint. Germanium junctions were certainly photo-sensitive. The OCP71 was more expensive, and people soon discovered that they could just scrape the paint off an OC71, as the glass capsule was originally filled with relatively transparent jelly-like material. Mullard later used an opaque filler, possibly to discourage this practice. OCP70 phototransistors in the same style can also be found, but they are usually unbranded, which I believe means that they are generic Philips devices for OEM use.

Of course, Mullard made diodes as well as transistors, and their OA series found widespread use. They made point-contact, gold-bonded, and junction types, plus Zeners, in both germanium and silicon,. This included subminature axial types, miniature axial types such as the OA81 often used as the AF detector in radio circuits, and switching types like the OA10 in the SO-2 glass package. Most of these diodes were painted black, but sometimes yellow was used. Later types used the AA (germanium) and BA (silicon) prefixes in their part numbers. Zeners had a 'Z' in the prefix, e.g., OAZ205.

This device is in the same primitive stud package with leads out of the top used for the prototype OC16 in about 1955. Disintegrating greased paper labels are marked "Semiconductor Lab. Development Sample. No guarantee for subsequent delivery." The label should bear an added written type number, but this is either missing or too faint.

This transistor CXT6b is a real puzzle. It is in the stud package used for the commercial OC16, the first Mullard power transistor, which suggests that it dates from about 1956. The only information that I have unearthed is that the 1963 CV Register of Electronic Valves mentions two GPO types, CXT1 and CXT2, with CV equivalents. This suggests that this CXT6b may be a GPO type, probably equivalent to OC16. However, GPO types are usually found in the PO series, itself rare. If anyone knows about this device or the CXT series, I would be exteremely interested to hear from them.

You may come across vintage GEC (UK) transistors and diodes bearing the Mullard name, such as the GET116 shown. This because, in 1961, GEC merged its semiconductor interests with those of Mullard Ltd., to form a joint venture called Associated Semiconductor Manufacturers (ASM). Devices were marketed through Mullard, and so GEC devices can be found bearing the Mullard name. In fact, ex-employees have told me that the 'joint venture' was a takeover in all but name.

Mullard Semiconductors